Pulp stabilisation apparatus for a thickener

ABSTRACT

The invention provides a flotation device including a thickening tank ( 1 ) having right cylindrical sidewalls and a downwardly sloping frusto-conical floor. A drive shaft ( 2 ) is centrally rotatably mounted within the tank. Each one of a plurality of rake arms ( 3 ) is fixedly connected to the drive shaft and extends radially outwardly, substantially parallel to the tank floor. Each arm supports an array of blades ( 4 ) extending axially into the tank. An array of baffles in the form of rectilinear baffle plates ( 5 ), fixed relative to the tank, extends axially into the tank in complementary alternate radially spaced relationship with the blades ( 4 ). The baffle plates are located within the swept volume of the blades to retard fluid rotation in response to the rotation of the rake. A diametrical service bridge ( 6 ) is fixedly connected to the tank for mounting the baffle plates.

FIELD OF THE INVENTION

The present invention relates to separation devices for liquidsuspensions and pulps and in particular to an improved thickener. It hasbeen developed primarily for use in separating mineral pulp from aliquid suspension and will be described hereinafter with reference tothis application. However, it will be appreciated that the invention isnot limited to this particular field of use.

BACKGROUND OF THE INVENTION

The following discussion of the prior art is intended to present theinvention in an appropriate technical context and allow its significanceto be properly appreciated. Unless clearly indicated to the contrary,however, reference to any prior art in this specification should not beconstrued as an admission that such art is widely known or forms part ofcommon general knowledge in the field.

Thickeners are known as a means of separating suspended pulps. They arecommonly used in the field of mineral processing and extraction, as wellas other industries such as paper production.

Known thickeners typically include a thickening tank and a feedwell. Thefeedwell includes a chamber having an inlet for receiving feed liquidand an outlet in fluid communication with the tank. In use, flocculationtakes place in the tank, whereby pulp of higher relative density tendsto settle towards the bottom of the tank, forming a bed of thickenedpulps and dilute liquor of lower relative density is thereby displacedtowards the top of the tank.

The feedwell is configured to reduce the turbulence of the incoming feedstream, to allow flocculants to be mixed into the liquid, and to allowsufficient residence time for reaction between flocculants and reagentsbefore the treated liquid is discharged into the thickening tank. Theconfiguration of the feedwell also promotes even distribution of liquidflowing from its outlet into the thickening tank.

The thickening tank may include a rotatably mounted rake. The rake istypically mounted on a centrally located drive shaft, driven by a motorand an associated gearbox. Each one of a plurality of radially extendingrake arms is fixedly connected to the rake and supports an array offixedly connected rake blades. The blades extend axially into the tanksuch that as the rake rotates, the blades sweep through the fluid in thetank. This sweeping shears the pulp particles, releasing trapped water,which rises to the surface through the sheared material, allowing thesolids to settle into a more dense bed of thickened pulp. This processis referred to in the art as “dewatering”. The sweeping also rakes thesettled solids to the centre of the tank from where they are discharged.

In contrast to its intended purpose, however, this rake in practicetends to push the whole or part of the settled material around with it,thus preventing it from performing its functions of assisting in thedewatering process and raking the settled solids to the centraldischarge point. This condition is referred to in the art as “donutting”and impedes the formation of the desired bed of relatively uniform,thickened pulp.

It is an object of the invention to overcome or substantially ameliorateone or more of the deficiencies of the prior art, or at least to providea useful alternative.

DISCLOSURE OF THE INVENTION

According to the invention, there is provided a separation deviceincluding a generally cylindrical thickening tank, and a rake rotatablymounted within said tank, said rake including a plurality of radiallyextending rake arms each including an array of spaced apart bladesextending into said tank, the separation device further including bafflemeans disposed to retard rotation of fluid within said tank in responseto rotation of said rake.

Preferably, the separation device is adapted for use as a thickener formineral suspensions and pulps.

Preferably, the baffle means includes an array of baffles fixed withrespect to the tank and extending generally axially into the tank.

Preferably, the baffles are positioned intermediate respective pairs ofblades, such that the baffles and blades are predominantly disposed inalternate radially spaced relationship.

Preferably, the baffles take the form of generally rectangular baffleplates located between adjacent rake blades and within a swept volume ofthe rake.

In another embodiment, the baffles may be formed from generallytriangular or other geometrically shaped plates complementary tosimilarly shaped rake blades formed from bars, wire, sheet, or plate.

In a further embodiment, the baffles take the form of an effectivelycontinuous barrier, provided by baffle formations including chains,bars, cables or ropes extending generally diametrically across saidtank.

Preferably also, the rake is located generally adjacent the tank floorand the blades extend generally upwardly towards the fluid surface. Inthis case, the baffles are preferably positioned above the rake andextend downwardly through the swept volume.

In one preferred embodiment, the device includes a bridge fixed relativeto and extending diametrically across the thickening tank and the baffleplates are fixedly mounted so as to extend downwardly into the sweptvolume, from the bridge.

According to a third aspect, the invention provides a method of reducingdonutting within a thickening tank in response to rotation of a rakeincluding a plurality of radially extending rake arms each including anarray of spaced apart blades extending into said tank, said methodincluding the step of providing baffle means disposed to retard rotationof fluid within said tank in response to rotation of said rake.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a plan view of an separation device according to theinvention; and

FIG. 2 is a sectional view of the device taken along line 2—2 of FIG. 1.

PREFERRED EMBODIMENTS OF THE INVENTION

A preferred application of the invention is in the field of mineralprocessing, separation and extraction, whereby finely ground ore issuspended as pulp in a suitable liquid medium, such as water, at aconsistency which permits flow, and settlement in quiescent conditions.The pulp is precipitated from the suspension by a combination ofchemical and mechanical processes. Initially, a flocculating agent isadded and mixed into the suspension. The suspension is then carefullymixed to facilitate the clumping together of solid particles, eventuallyforming larger denser particles that are precipitated out of suspension.

Referring to the drawings, the device includes a thickening tank 1having right-cylindrical sidewalls and a downwardly slopingfrusto-conical floor. A drive shaft 2 is centrally rotatably mountedwithin the tank. Each one of a plurality of rake arms 3 is fixedlyconnected to the drive shaft and extends radially outwardly,substantially parallel to the tank floor. Each arm supports an array ofblades 4 extending axially into the tank. An array of baffles in theform of rectilinear baffled plates 5, fixed relative to the tank,extends axially into the tank in complementary alternate radially spacedrelationship with the blades 4. The baffle plates are located within theswept volume of the blades to retard fluid rotation in response to therotation of the rake. A diametrical service bridge 6 is fixedlyconnected to the tank for mounting the baffle plates. The separationdevice also has a feedwell 11 including a feedwell tank located near thetop of the thickening tank 1 and above the rake arms 3.

In use, the rake 2 rotates and the blades 4 pass through the respectiveclearance spaces provided between adjacent baffle plates, gentlysweeping through the fluid to shear the pulp particles, allowing therelease of trapped water, which rises to the surface through the shearedmaterial. This, in turn, allows the solids to settle into a more densebed of thickened pulp 7. The baffle plates 5 retard rotation of the bedof thickened pulp 7 during the sweeping motion of the rake, therebyfacilitating the shearing process. The thickened pulp displaces an upperlayer of liquid 8, which is progressively drawn off through an overflowlaunder 9, extending circumferentially around the tank. The thickenedpulp 7, on the other hand, is drawn off from below through an underflowwithdrawal pipe 10 included at the base of the tank.

It will be appreciated that the baffles of the illustrated device reducethe tendency of the rake to push the thickened pulp around the tank. Asa result, the problem of the rake breaking up, or donutting, thethickened pulp is substantially reduced, thereby increasing theefficiency of the pulp separation process. In this respect, theinvention represents a practical and commercially significantimprovement over the prior art.

Although the invention has been described with reference to a specificexample, it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms.

1. A separation device including a generally cylindrical thickeningtank, a feedwell for receiving feed liquid, and a rake for dewatering abed of thickened pulp in said thickening tank, said feedwell including afeedwell tank located near the top of the thickening tank and above therake, said rake being rotatably mounted within said thickening tank andincluding a plurality of radially extending rake arms each including anarray of spaced apart blades extending into said thickening tank, theseparation device further including baffle means disposed to retardrotation of thickened pulp within said thickening tank in response torotation of said rake.
 2. A separation device according to claim 1,wherein the baffle means includes an array of baffles fixed with respectto said thickening tank.
 3. A separation device according to claim 2,wherein said baffles extend generally axially into said thickening tank.4. A separation device according to claim 2, wherein the baffles arepositioned intermediate respective pairs of said blades, such that thebaffles and blades are predominantly disposed in alternate radiallyspaced relationship.
 5. A separation device according to claim 2,wherein the baffles take the form of generally rectangular baffle plateslocated between adjacent rake blades and within a swept volume of therake.
 6. A separation device according to claim 2, wherein the bafflesare formed from geometrically shaped plates complementary to similarlyshaped rake blades.
 7. A separation device according to claim 1, whereinthe blades are formed from bars, wire, sheet, or plate.
 8. A separationdevice according to claim 1, wherein the rake is located generallyadjacent the thickening tank floor and the blades extend generallyupwardly.
 9. A separation device according to claim 8, wherein thebaffle means is positioned above the rake and extends downwardly throughthe volume swept by the rake.
 10. A separation device according to claim1, including a bridge fixed relative to and extending diametricallyacross the thickening tank.
 11. A separation device according to claim10, wherein the baffle means is fixedly mounted to the bridge andextends downwardly into the volume swept by the rake.
 12. A method ofreducing donutting within a thickening tank, said thickening tank havinga feedwell for receiving feed liquid and a rake for dewatering a bed ofthickened pulp, said feedwell including a feedwell tank located near thetop of the thickening tank and above the rake, said rake including aplurality of radially extending rake arms each including an array ofspaced apart blades extending into said thickening tank, said methodincluding the step of providing baffle means disposed to retard rotationof thickened pulp within said thickening tank in response to rotation ofsaid rake.
 13. A method according to claim 12, for use in a process forextracting solids from mineral suspensions and pulps.
 14. A methodaccording to claim 12, wherein the baffle means includes an array ofbaffles fixed with respect to said thickening tank.
 15. A methodaccording to claim 14, wherein said baffles extend generally axiallyinto said thickening tank.
 16. A method according to claim 14, includingthe step of positioning the baffles intermediate respective pairs ofsaid blades, such that the baffles and blades are predominantly disposedin alternate radially spaced relationship.
 17. A method according toclaim 14, wherein the baffles take the form of generally rectangularbaffle plates located between adjacent rake blades and within a sweptvolume of the rake.
 18. A method according to claim 14, wherein thebaffles are formed from geometrically shaped plates complementary inshape to the rake blades.
 19. A method according to claim 12, whereinthe blades are formed from bars, wire, sheet, or plate.
 20. A methodaccording to claim 12, wherein the rake is located adjacent thethickening tank floor and the blades extend generally upwardly.
 21. Amethod according to claim 20, wherein the baffle means is positionedabove the rake and extends downwardly through the volume swept by therake.
 22. A method according to claim 12, including the further step ofproviding a bridge fixed relative to and extending diametrically acrossthe thickening tank and supporting the baffle means from the bridge. 23.A method according to claim 22, wherein the baffle means are fixedlymounted to the bridge and extend generally downwardly into the volumeswept by the rake.